Using macOS 10.13.3. In order to access previous returns I'm asked to enter "key chain password" but I've never used this and don't know how to access it. What to do?

Your keychain password should be the password you use to log on to your Mac user account.  You are giving the program permission to access old files by inputting your password to prove that the person sitting at the keyboard now is the authorized user.

Thank you!  So simple!